As the technology of the light-emitting diodes (LEDs) develops, the light-emitting diodes are applied widely. And now a light-emitting device adopting the light-emitting diode usually comprises more than one single light-emitting diode. For example, the light-emitting device such as a High Voltage Light-Emitting Diode (HVLED), an Alternating Current Light-Emitting Diode (ACLED), or an Array that is commonly used as a display, a traffic sign, and a lighting, etc., comprises a plurality of light-emitting diodes. Taking the HVLED as an example, as a single light-emitting diode works at a low voltage, the HVLED is formed by a plurality of light-emitting diodes connected in series. FIG. 1 shows an HVLED. The externally supplied AC voltage 11 is reduced in its voltage level by a converter 12 and converted into a DC voltage corresponding thereto. Then the converted DC voltage is fed into a plurality of light-emitting diodes. As shown in FIG. 1, to form the HVLED, a light-emitting diode is connected with one another in series. And then each series is connected in parallel with other series. This can be done by a chip level design to layout the plurality of light-emitting diodes in a single chip, or by a package level to have the plurality of light-emitting diodes in a package. However, some types of failures caused by the manufacturing process or material defects, such as the current leakage in a small local area or a surface stain/damage which causes the light shaded, cannot be detected by a conventional electrical test. For a light-emitting device comprises a plurality of light-emitting diodes connected in series and/or parallel, if only a few light-emitting diodes fail, it is difficult to be detected by a conventional electrical test. Besides, the uniformity of the luminous intensity of the plurality of light-emitting diodes which often concerns the application cannot be measured in the conventional electrical test as well.